Deformable image registration phantom

ABSTRACT

A deformable image registration phantom having a housing, an outer cylinder, a parallel eccentric inner cylinder, a ball and socket mount, and a target. The inner cylinder is rotatably mounted within the outer cylinder and the target is rotatably mounted, directly or indirectly, within the inner cylinder. Indexing detents are positioned on at least one of the housing, the outer cylinder, the inner cylinder, and the ball and socket mount and one or more detent pins positioned on another. The indexing detents are arranged about at least one rotational or translational axis of the phantom and the detent pins are configured to releasably engage with the indexing detents as the components of the phantom rotate or translate relative to one another.

FIELD OF THE INVENTION

The present invention relates to an imaging quality assurance (QA) phantom apparatus and, in particular, to an adjustable deformable image registration phantom for on-table adaptive radiotherapy including dosimetry and dose accumulation.

BACKGROUND

Radiation treatment patients exhibit movement and deformation in many degrees of freedom, due to processes including weight loss, tumour shrinkage, inflammation, motion due to respiration, or simply lying in a slightly different position between treatment sessions. This presents challenges for radiation treatment in delivering the radiation dose to the target while minimizing the damage to surrounding healthy tissue or organs at risk.

Deformable image registration (DIR) is the process of finding correspondence between images that are not linked by simple translational shifts or rotations. This can be applied to many types of imaging, such as commutated tomography (CT) imaging or magnetic resonance imaging (MRI), and used in verification and adaptive treatment planning. The ability to model and adapt treatment plans for physiological changes, such as weight loss over the course of the treatment plan, will help clinicians reduce treatment margins, increase dose to the tumour, and decrease dose to healthy tissue. This permits more effective treatment of cancers associated with high mortality due to their proximity to other sensitive or moving organs, such as pancreatic, liver, and lung cancer.

The use of DIR in verifying and adapting treatment plans requires image quality assurance tools that are capable of producing quantified movements (both translational and rotational) and deformation. The present invention relates to the design of a DIR phantom capable of movement with six degrees of freedom and deformation, to verify the accuracy of the DIR process in treatment planning.

SUMMARY OF THE INVENTION

A deformable image registration phantom, according to the present invention, has a housing, an outer cylinder having a first diameter, a parallel eccentric inner cylinder having a second diameter smaller than the first diameter, a ball and socket mount, and a target. The target is mounted to the housing by way of the inner and outer cylinders and the ball and socket mount. The inner cylinder is rotatably mounted within the outer cylinder and the target is rotatably mounted, directly or indirectly, within the inner cylinder. A plurality of indexing detents are positioned on at least a first component selected from a group consisting of the housing, the outer cylinder, the inner cylinder, and the ball and socket mount. One or more detent pins positioned on at least a second component selected from the group consisting of the housing, the outer cylinder, the inner cylinder, and the ball and socket mount. The plurality of indexing detents are arranged about at least one rotational or translational axis of the phantom and the one or more detent pins are configured to releasably engage with the plurality of indexing detents as the first and second components rotate or translate relative to one another.

In another embodiment, the ball and socket mount is rotatably mounted within the housing and has a key extending from the ball and socket mount that fits within a complementary-shaped keyway in the housing, and the key is limited to movement within the keyway in the rotational pitch axis of the phantom.

In another embodiment, the housing has a base with a rotating platform that rotates in the yaw rotational axis of the phantom, and wherein the ball and socket mount is mounted on the rotating platform.

In another embodiment, at least some of the plurality of indexing detents are positioned about the perimeter of the inner cylinder and at least one of the one or more detent pins is positioned to engage with the indexing detents positioned about the perimeter of the inner cylinder.

In another embodiment, three detent pins are positioned to engage with the indexing detents positioned about the perimeter of the inner cylinder and the indexing detents positioned about the perimeter of the inner cylinder are arranged in three rings, each comprising 120 indexing detents arranged at 3° increments. Each of the three detent pins are positioned over one of the rings and the indexing detents in each of the rings are offset from the indexing detents in each of the other rings, such that one indexing detent about the perimeter of the inner cylinder is positioned at every degree increment about the perimeter of the inner cylinder.

In another embodiment, the deformable image registration phantom has a housing, one or more components that move rotationally or translationally relative to the housing or another one of the one or more components, a target mounted to the housing by way of the one or more components, a plurality of indexing detents positioned on at least a first component selected from a group consisting of the housing and the one or more components, and one or more detent pins positioned on at least a second component selected from the group consisting of the housing and the one or more components. The plurality of indexing detents are arranged about at least one rotational or translational axis of the phantom and the one or more detent pins are configured to releasably engage with the plurality of indexing detents as the first and second components rotate or translate relative to one another. The one or more components are rotatably or translatably mounted within the housing and the target is rotatably or translatably mounted directly or indirectly within the one or more components.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, a preferred embodiment thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is an isometric view of the deformable image registration phantom, according to the present invention.

FIG. 2 is a front view of the phantom of FIG. 1 .

FIG. 3 is a side view of the phantom of FIG. 1 .

FIG. 4 is a side-sectional view of the phantom of FIG. 1 , along the lines A-A as shown in FIG. 2 .

FIG. 5 is a front-sectional view of the phantom of FIG. 1 , along the lines B-B as shown in FIG. 3 .

FIG. 6 is a rear view of the phantom of FIG. 1 .

FIG. 7 is an exploded isometric view of the phantom of FIG. 1 .

FIG. 8 is an isometric view of another embodiment of the deformable image registration phantom, according to the present invention.

FIG. 9 is a front view of the phantom of FIG. 8 .

FIG. 10 is a side view of the phantom of FIG. 8 .

FIG. 11 is a side-sectional view of the phantom of FIG. 8 , along the lines C-C as shown in FIG. 9 .

FIG. 12 is a front-sectional view of the phantom of FIG. 8 , along the lines D-D as shown in FIG. 10 .

FIG. 13 is a rear view of the phantom of FIG. 8 .

FIG. 14 is an exploded isometric view of the phantom of FIG. 8 .

FIG. 15 is another exploded isometric view of the phantom of FIG. 8 .

FIG. 16 is an isometric view of yet another embodiment of the deformable image registration phantom, according to the present invention.

FIG. 17 is a front view of the phantom of FIG. 16 .

FIG. 18 is side view of the phantom of FIG. 16 .

FIG. 19 is a rear sectional view of the phantom of FIG. 8 , along the lines E-E as shown in FIG. 18 .

FIG. 20 is an exploded isometric view of the phantom of FIG. 16 .

FIG. 21 is another isometric view of the phantom of FIG. 16 , with part of the base removed to show the indexing detents for the yaw rotational axis.

FIG. 22 is a rear isometric view of the phantom of FIG. 16 , with part of the housing removed to show the indexing detents for the pitch rotational axis.

FIG. 23 is a detail view of the indexing detents for the pitch rotational axis, as shown in FIG. 22 .

FIG. 24 is a detail view of the phantom of FIG. 16 , with part of the base removed to show the indexing detents for the front-rear translational axis.

FIG. 25 is a detail view of the phantom of FIG. 16 , showing the indexing detents for the rotation of the inner and outer cylinders.

FIG. 26 is another detail view of the phantom of FIG. 16 , as shown in FIG. 25 , with the cover removed to show the detent pins.

FIG. 27 is an isometric view of the detent pins and cover for the rotation of the inner and outer cylinders.

FIG. 28 is another isometric view of the detent pins and cover of FIG. 27 , showing the detent pins in broken lines through the cover.

FIG. 29 is a front view of the detent pins and cover of FIG. 27 .

FIG. 30 is a side-sectional view of the detent pins and cover of FIG. 27 , along the lines F-F as shown in FIG. 29 .

DESCRIPTION OF THE INVENTION

The deformable image registration phantom, according to the present invention, permits controlled adjustment of the position of an imaging target in an imaging phantom with six degrees of freedom. A deformable imaging target may also be used to simulate any kind of movement or deformation of a target. The present invention may be used to test the accuracy of a piece of imaging equipment, such as a computerized tomography (CT) or magnetic resonance imaging (MRI) device, in tracking known movements and deformations of a target. This permits quality assurance of imaging equipment used for pre-treatment adjustments to radiation treatment plans. The target may also include one or more radiation dose detectors to permit confirmation of radiation dose delivery to the target, according to the adjusted treatment plan.

As shown in FIGS. 1-7 , the phantom 1 has a housing 2, two parallel eccentric cylinders 3 and 4, a ball and socket mount 5, and a target 6. The housing 2 may have any desired shape or configuration, for example, a cylinder, a truncated cylinder or oval, or an anatomical shape, such as a torso, a skull, an abdomen, or a thorax. Preferably, the housing 2 is a truncated cylinder or oval to provide a flat, stable base for the phantom 1. The parallel eccentric cylinders comprise an outer cylinder 3 and an inner cylinder 4. The outer cylinder 3 is rotatably mounted within a complimentary shaped opening in the housing 2, so as to rotate about a first axis, and has a first diameter d₁. The inner cylinder 4 has a second diameter d₂ that is smaller than the first diameter d₁, and is rotatably mounted within a complimentary shaped opening in the outer cylinder 3, so as to rotate about a second axis. The second axis is parallel to the first axis and is also offset from the first axis by an offset distance D₁.

As shown in FIGS. 2, 4, and 5 , the ball and socket mount 5 for the target 6 is mounted within the inner cylinder 4. The ball and socket mount 5 is positioned with its centre of rotation offset from the second axis of the inner cylinder 4 by an offset distance Dz. As a result, the target 6 (within the ball and socket mount 5) moves relative to the housing 2 when either or both the inner cylinder 3 and outer cylinder 4 are rotated. The ball and socket mount 5 permits rotation of the target 6 about three axes (pitch, roll, and yaw). The target 6 may be any desirable configuration or type of target, but is preferably a cylindrical insert 7 containing a deformable imaging target 6, which slides into position at the centre of rotation of the ball and socket mount 5. This permits rotation of the target 6 along three axes (pitch, roll, and yaw) without displacement of the target 6 (i.e. movement along the x, y, or z axes). Alternatively, the target 6 may be offset from the centre of rotation of the ball and socket mount 2. A plurality of inserts 7 with variously sized targets 6 may be used and substituted for one another to simulate changes in overall size of the target 6, for example to simulate the reduction or growth in size of a tumour. The plurality of inserts 7 may also include targets 6 with various imaging properties. Inserts 7 may also be used, which include a plurality of targets 6 to simulate multiple radiation targets or organs at risk, or both. The term “target” will be used to refer generally to both radiation targets and organs at risk, but any particular target may be one or the other, as needed.

As shown in FIGS. 1, 2, and 7 , the inner cylinder 4 has an opening through which the ball and socket mount 5 may be accessed for inserting or adjusting the position of the target 6. Preferably, the ball and socket mount 5 may be rotated unconstrained about the axis parallel to the second axis of the inner cylinder 4 (the roll axis). In configurations where an insert 7 protrudes from the ball and socket mount 5 so as to interfere with the free rotation of the ball and socket mount 5 about the two axes orthogonal to the second axis (the pitch and yaw axes), as shown in FIGS. 1 and 4 , the rotation of the ball and socket mount 5 about these axes is constrained by the size of the insert 7 and the opening in the inner cylinder 4.

As shown in FIGS. 2, 4, and 5 , the outer cylinder 3 and inner cylinder 4 may be rotated to a position where the offset distances D₁ and D₂ are oriented in opposite directions, such that the ball and socket mount 5 and target 6 are positioned at the centre of the phantom 1. This occurs when the offset distances D₁ and D₂ are equal in length. From that position, the target 6 may be displaced in the plane orthogonal to the first and second axes (i.e. on the x and y axes), by rotating either or both the outer cylinder 3 and the inner cylinder 4. Rotating the inner cylinder 4 within the outer cylinder 3 changes the distance between the centre of rotation of the ball and socket mount 5 and the first axis of the outer cylinder 3. Rotating the outer cylinder 3 within the housing 2 moves the centre of rotation of the ball and socket mount 5 along a circular path relative to the housing 2. By combining the rotation of the outer cylinder 3 and the inner cylinder 4, the centre of rotation of the ball and socket mount 5, and therefore the target 6, may be displaced to any location along the x and y axes within the area determined by the offset distances D₁ and Dz. Displacement in the z axis (the same axis as the first and second axes of the outer and inner cylinders 3 and 4) may be accomplished by sliding either the outer or inner cylinder 3 or 4, or where used, the insert 7, relative to other components of the phantom 1 in which they are mounted. Alternatively, the phantom 1 may be moved along the z axis on the imaging table in the imaging device.

The movement of the outer and inner cylinders 3 and 4 may be manually controlled using control knobs 8 mounted on and accessible from the front 2 a of the housing 2, as shown in FIGS. 2 and 3 . The front 2 a of the housing 2 may also have markings about the opening in which the outer cylinder 3 is mounted, to indicate the angle of rotation of the outer cylinder 3 relative to the housing 2. Similarly, the front 3 a of the outer cylinder 3 may have visual markings about the opening in which the inner cylinder 4 is mounted, to indicate the angle of rotation of the inner cylinder 4 relative to the outer cylinder 3. Visual markers may also be provided on the rear of the phantom 1, such as on the inner cylinder 4, as shown in FIG. 6 , or on the ball and socket mount 15, as shown in FIGS. 13, and 15 . Alternatively, or in addition to visual markers, imaging structures, such as fiducials 9, may be added to one or more of the housing 2, outer cylinder 3, inner cylinder 4, ball and socket mount 5, insert 7, or target 6 to facilitate measurement of their relative position with the relevant imaging equipment and track adjustments more accurately than may be possible with visual observation and manual measurement by an imaging technician.

Although the phantom 1 has been described as configured with the ball and socket mount 5 located within the inner cylinder 4 (as shown in FIGS. 1-7 ), the phantom 1 may alternatively be configured the other way around, with the outer cylinder 4 located within the ball and socket mount 5 (as shown in FIGS. 8-15 ). This alternative configuration operates similarly to the configuration described above, but the relative positioning and mounting of the components are different. The target 6, or insert 7 containing the target 6, is located within or mounted in the inner cylinder 14, rather than the ball and socket mount 5, since the inner cylinder 14 is the innermost of these three components, in this configuration. In either configuration, the target 6 is mounted, directly or indirectly, within the inner cylinder 4 or 14.

The inner and outer cylinders 14 and 13 are both structurally and functionally similar to the configuration of the inner and outer cylinders 4 and 3, as described above. In both cases, they are parallel eccentric cylinders with first and second axes offset by an offset distance D₁. However, as shown in FIGS. 11, 14, and 15 , the inner and outer cylinders 14 and 13 preferably have hemispherical rear ends 14 a and 13 a to fit and facilitate rotation within the ball and socket mount 5.

As shown in FIGS. 11, 12, 14, and 15 , the ball and socket mount 15 is preferably a two-component ball and socket mount that operates like a gimbal mount, with an inner gimbal 15 a and an outer gimbal 15 b. The inner gimbal 15 a has a third axis, about which it rotates, that is perpendicular to the first and second axes of the inner and outer cylinders 14 and 13. The outer gimbal 15 b has a fourth axis, about which it rotates, that is perpendicular to the third axis of the inner gimbal 15 a and also to the first and second axes of the inner and outer cylinders 14 and 13. As shown in FIG. 12 , the inner gimbal 15 a has a generally annular shape that fits within the outer gimbal 15 b and accommodates the outer cylinder 13. In particular, the inner gimbal 15 a has the shape of a spherical segment (or spherical frustum) with a hemispherical cylinder-shaped aperture through its centre. The outer gimbal 15 b also has a generally annular shape to fit within the housing 12 and accommodate the inner gimbal 15 a. Like the inner gimbal 15 a, it has the shape of a spherical segment, but with a spherical segment-shaped aperture through its centre. Preferably, as shown in FIGS. 14 and 15 , the inner gimbal 15 a is formed in a single piece, while the outer gimbal 15 b is formed in two pieces to facilitate ease of assembly.

As best illustrated in FIGS. 14 and 15 , the inner gimbal 15 a has posts 16 that extend outwardly from the inner gimbal 15 a and engage with the outer gimbal 15 b to restrict the movement of the inner gimbal 15 a relative to the outer gimbal 15 b to rotation about the third axis. Preferably, the posts 16 are no longer than the thickness of outer gimbal 15 b, so as not to interfere with the movement of the outer gimbal 15 b relative to the housing 12. Similarly, the outer gimbal 15 b has posts 17 that extend outwardly from the outer gimbal 15 b and engage with the housing 12 to restrict movement of the outer gimbal 15 b relative to the housing 12 about the fourth axis. Preferably, the third and fourth axes are configured so that the rotation of the inner gimbal 15 a adjusts the pitch, while the rotation of the outer gimbal 15 b adjusts the yaw. Where the phantom 11 uses visual markers to indicate the relative position of the various components, the inner and outer cylinders 14 and 13 as well as the post 17 and the hemispherical rear end 13 a of the outer cylinder 13 may marked with visual indicators to indicate the precise relative position of each component. In this case, the post 17 and the hemispherical rear end 13 a are visible on the outside of the phantom 11, as shown in FIGS. 8, 13, and 15 .

The phantom 1 may be configured for any desired imaging modality, such as X-ray, CT, PET, or MRI. Preferably, the phantom 1 and its components are made of a solid, rigid material for X-ray or CT imaging (kV or MV). Alternatively, the phantom 1, or one or more of its components, is configured for MM or PET imaging and is made of a rigid (MRI invisible) material, containing one or more sealed reservoirs containing a fluid (MM signal producing) material. Where the phantom 1 contains a sealed reservoir, it is preferably configured with a first and second volume portion, as described in U.S. Pat. Nos. 10,180,484 B2 or 10,310,048 B2 in the name of the present applicant. The first volume portion is the space defined by the side wall and the end caps of the housing 2. The second volume portion is an expandable chamber or an expandable bladder attached to the housing 2 and in fluid communication with the first volume portion.

For MRI applications, the housing 2 is preferably made of acrylic, as it has closely matched susceptibility to human tissue, and the MRI signal producing material is an MRI contrast medium, such as aqueous solution with a close susceptibility match to human tissue. The aqueous solution is doped with one or more suitable T1 and T2 relaxivity modifiers, adjusted to give physiologically relevant T1 and T2 values with specific contrast between healthy tissue and tumors. Alternatively, the housing may be another human tissue equivalent susceptibility-matched plastic and the MM contrast media may be an aqueous solution with added viscosity modifiers, mineral oil, silicone oil, vegetable oil, propylene glycol, or a gel that produces an MRI signal. Preferably, the relaxivity modifier is one or more of: copper (II) sulfate (CuSO₄), manganese (II) chloride (MnCl₂), gadolinium (III) chloride (GdCl₃), or other salts and chelates of paramagnetic metals that are soluble or freely dispersed in the MM contrast media, superparamagnetic iron oxide nanoparticles (SPIONs), or micelles. Preferably, the phantom 1 is configured for multimodality applications, including two or more of: MRI, CT (MV or kV), and PET imaging.

In addition to multimodality imaging applications, the phantom 1 may be configured for use in dosimetry, gel dosimetry, deformable dose accumulation, deformable image registration (DIR), deformable image fusion, targeting, or gating applications. Preferably, in applications where dosimetry is desirable, the insert 7 contains one or more radiation dose detectors, such as ion chambers, diodes, OSLDs, scintillators, MOSFETs, diamond detectors, film diodes or chamber arrays, 3D gel or solid dosimeters. Preferably, the insert 7 contains a target 6 that is a gel imaging target or a gel-filled target for 3D imaging and dosimetry and is also deformable. Such deformable dosimeters can be formed by addition of radiation indicators, such as acrylamide and bis-acrylamide monomers, to a deformable hydrogel, where the radiation-induced polymerization can be determined by MRI, CT or optical methods. Other suitable indicators include radiochromic dyes, which may be added to a deformable hydrogel or to urethane or silicone rubber materials, where the radiation-induced color change can be determined by optical methods. Alternatively, the radiation-induced cross-linking of silicone or urethane rubbers, doped with appropriate additives, may be detected by MRI or CT imaging and used to measure the accumulated radiation dose. In the further alternative, an ion chamber or other radiation measurement device may be placed in a recess in the target 6, which may act as or receive an ion chamber holder.

The insert 7 may have the structure substantially similar to the deformable imaging phantom described in United States patent application publication US2020/0400716 A1, “Deformable Imaging Phantom for 4D Motion Tracking”, by the present applicant. In such deformable applications, the insert 7 has a generally cylindrical shape, with a continuous side wall and two opposing end caps. The side wall and end caps define a sealed reservoir filled with an MRI contrast media and a deformable target 6. Preferably, the deformable target 6 is made of an open cell polyurethane foam, but other similar materials may be used, such as open cell poly(vinyl alcohol) foam, open cell silicone foam, closed cell foams, other foams, or other viscoelastic materials. Alternatively, continuous materials with viscoelastic properties and intrinsic MRI signal, such as urethane rubbers, silicone rubbers, or thermoplastic elastomers (such as styrene-ethylene-butylene-styrene co-polymer) may be used on viscoelastic hydrogels, based on natural or artificial gel forming polymers, such as gelatin, agarose, poly(vinyl alcohol), acrylamide-based polymers, or combinations thereof, with or without cross-linking agents, such as metal ion salts, aldehydes, amines, or acrylamides. Preferably, the deformable target 6 is in free contact with the MRI contrast media, which fills the sealed reservoir inside the insert 7 and the interstitial spaces in the deformable target 6, if any. Alternatively, the deformable target 6 may be enclosed in a thin-walled barrier, or coating, of deformable material, such as natural or artificial rubber, silicone, fluorosilicone rubber, or similar inert elastomers. Where a foam material is used, it is preferably impregnated with MR contrast media using a vacuum for air bubble removal to minimize MR and CT imaging artifacts in the target zone. The material is not limited to foam and could be a urethane rubber, thermoplastic elastomer (such as styrene-ethylene-butylene-styrene co-polymer), latex balloon, or hydrogel, with or without the addition of high-density, non-metallic, non-conductive powder for increased CT contrast. The different proton or electron densities of these regions results in different imaging properties. In FIG. 1 , the target 6 is illustrated as a single off-centre ellipsoidal structure, but may have other positions and geometries, such as a spherical shape or the physiological shape of an organ or tumour.

Fiducial markers, or fiducials 9, may be positioned at any desired location within the phantom 1 for modeling verification, such as attached to the target 6 within the deformable insert 7, as shown in FIG. 1 . The fiducials 9 may be additive or subtractive, positive or negative signal. Preferably, spherical fiducials 9 at least 3 mm in diameter are fixed to the target 6 and positioned on three orthogonal axes. The fiducials 9 provide verification for interpreting the position and motion of the target 6 on kV or MV CT imaging, for example. Where the fiducials 9 are intended to be used in MV CT applications, they must be of a sufficiently high-density material for visibility, such as an alumina or zirconia ceramic material. Alternatively, the fiducials 9 may be made of another high-density non-metallic material that is visible on MV. In a further alternative, fiducials 9 may not be necessary where the target 6, itself, has a density such that it is visible on kV and MV imaging. Fiducials may also be unnecessary for MR-only applications.

A motion assembly, such as a MR compatible motor system, may be connected to the insert 7 to drive the motion of the insert 7 relative to the housing 2 or to drive the deformation of a deformable target 6, or both. Preferably, the motion assembly is a MR compatible piezoelectric motor assembly, as described in U.S. Pat. No. 10,090,781 B2 in the name of the present applicant. Alternatively, other types of motion assemblies may be used, such as pneumatic or hydraulic drives.

Another embodiment of the phantom 21, as shown in FIGS. 16-30 , uses a different configuration of the housing 22 and the ball and socket mount 25, and has a plurality of indexing detents 30 that engage with a plurality of detent pins 31 to facilitate greater control over the translational and rotational position of the moving parts of the phantom 21. Otherwise, the phantom 21 has outer and inner cylinders 23 and 24 that are configured and operate as described above. Although described below in relation to the phantom 21 shown in FIGS. 16-30 , the indexing detents 30 and detent pins 31 may be used with any imaging phantom or insert that includes one or more components that move rotationally or translationally relative to a housing or one another in order to facilitate greater control over the movement and position of the moving components of the phantom or insert.

As shown in FIGS. 16-22 , the housing 22 is a C-shape or clamshell-shape that extends from the bottom, around the rear, and over the top of the ball and socket mount 25 to hold it in place for mounting on an MRI system patient table or other imaging platform. The front 22 a of the housing 22 is open to permit access to the outer and inner cylinders 23 and 24. Preferably, the sides of the housing 22 are also open to minimize potential pinch points around the front 22 a where an operator will generally be gripping the phantom 21 to reposition the various moving parts to the desired position. The housing 22 may have a base 26, with attachment points or other features to engage with or lock to complementary structures on the imaging platform. The housing 22 may also have a viewing window 27 to allow an operator to view visual markers on the top of the phantom 21. As shown in FIG. 16 , the viewing window 27 has a sideways T-shape to permit viewing visual markers for both the pitch rotation and yaw rotation of the phantom 21. These may be similar to the visual markers surrounding the outer and inner cylinders 23 and 24, as described above.

As shown in FIGS. 18-23 , the ball and socket mount 25 is rotatably mounted within the housing 22. Preferably, the ball and socket mount 25 is permitted to rotate about the pitch and yaw axis relative to the housing 22. Rotation about the roll axis is permitted by rotation of the insert 7, where one is used, or by rotation of the outer and inner cylinders 23 and 24 (which also permit translation about the left-right and up-down translational axis). As shown in FIGS. 19, 22, and 23 , rotation about the pitch axis is permitted by a key 27 that extends from the ball and socket mount 25 and fits within a complementary-shaped keyway 28 in the housing 22. The keyway 28 is an arc-shaped channel positioned adjacent to the lower-rear of the ball and socket mount 25 and is shaped to receive the key 27 and allow it to slide within the keyway 28, but otherwise restrict the relative movement of the ball and socket mount 25 with the portion of the housing including the keyway 28. Preferably, the base 26 of the housing 22 includes a rotating platform 26 a on which the ball and socket mount 25 is attached (by way of the key 27 and keyway 28). The rotating platform 26 a is rotatably mounted within the base 26 and permits the ball and socket mount 25 to rotate about the yaw axis relative to the rest of the housing 22.

As shown in FIGS. 19 and 21-30 , a plurality of indexing detents 30 are positioned at various positions about the phantom 21 and engage with complementary-shaped detent pins 31 to facilitate greater control over the rotational and translational movement of the moving parts of the phantom 21. The detent pins 31 are biased towards their respective indexing detents 30 by way of springs 32. Preferably, the indexing detents 30 are arranged at 1° increments about the rotating parts of the phantom 21 and at 1 mm increments about the translating parts of the phantom. The use of indexing detents 30 and detent pins 31 provides greater precision in positioning and also haptic feedback to an operator when grasping and repositioning the moving parts of the phantom 21. Rather than relying on the alignment of visual markers on the outside of the phantom 21, an operator can feel the resistance of the springs 32 of the detent pins 31 as the phantom 21 moves about the corresponding rotational or translational axis.

The detent pins 31 that are used with the indexing detents 30 about the inner cylinder 24 are shown, for illustration, in detail views in FIGS. 27-30 and will be discussed below, by way of example. However, various configurations of detent pins may be used with the indexing detents 30 on the phantom 21, provided that they are made of MR-compatible materials where the phantom 21 is intended for use inside a MRI device. As shown in FIGS. 25 and 26 , the inner cylinder 24 has a plurality of indexing detents about the perimeter of its exposed portion at the front 22 a of the housing 22. Positioned above each ring of indexing detents 30 is a detent pin 31 mounted within a cover 31 a extending from the outer cylinder 23. Preferably, there are 120 indexing detents 30 in each of three rings about the perimeter of the inner cylinder 24, with each indexing detent 30 in each ring arranged at increments of 3° of rotation from each adjacent indexing detent 30 about the roll axis of the phantom 21. The indexing detents 30 in each of the second and third rings are offset from the first by 1° and 2°, so that one of the detent pins 31 will be seated in one of the indexing detents 30 in one of the rings at each 1° increment of rotation about the roll axis.

As shown in FIGS. 28 and 30 , each detent pin 31 has a spring 32, preferably comprised of a series of conical washers 33 arranged into pairs facing one another in a stack. FIG. 30 shows each spring 32 having four conical washers 33, but any desirable number of conical washers 33 may be used to suit the requirements of the application, such as travel distance of the detent pin 31. Each conical washer 33 is preferably made of an elastic MR-safe material, so as to provide a spring action when the detent pin 31 is pressed against the spring 32 as it moves between indexing detents 30. The detent pins 31 and spring 32 are positioned within a complementary-shaped recess in the cover 31 a (or elsewhere in the phantom 21) that is positioned over their corresponding indexing detents 30.

As shown in FIGS. 19, 22, and 23 , indexing detents 30 may be positioned within the keyway 28 and detent pins 31 positioned on the key 27, to provide indexing on the pitch rotational axis of the phantom 21. Similarly, as shown in FIG. 24 , indexing detents 30 and detent pins 31 may be positioned on the base 26 to provide indexing on the front-rear translational axis of the phantom 21. Preferably, the indexing detents 30 are arranged at 1 mm increments from each other on a portion of the base 26 that slides in the front-rear translational axis relative to the portion of the base that is mounted on the imaging table, in which the corresponding detent pins 31 are mounted. Similarly to the indexing of the inner cylinder 24, as described above, indexing detents 30 are positioned about the perimeter of the outer cylinder 23 and corresponding detent pins 31 may be positioned within the ball and socket mount 25. In this way, indexing may be provided on all three rotational axis, as well as the front-rear translational axis of the phantom 21.

The present invention has been described and illustrated with reference to an exemplary embodiment, however, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as set out in the following claims. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed herein. 

What is claimed is:
 1. An deformable image registration phantom, comprising: a housing; an outer cylinder having a first diameter; a parallel eccentric inner cylinder having a second diameter smaller than the first diameter; a ball and socket mount, a target mounted to the housing by way of the inner and outer cylinders and the ball and socket mount; a plurality of indexing detents positioned on at least a first component selected from a group consisting of the housing, the outer cylinder, the inner cylinder, and the ball and socket mount; and one or more detent pins positioned on at least a second component selected from the group consisting of the housing, the outer cylinder, the inner cylinder, and the ball and socket mount; wherein the plurality of indexing detents are arranged about at least one rotational or translational axis of the phantom and the one or more detent pins are configured to releasably engage with the plurality of indexing detents as the first and second components rotate or translate relative to one another; and wherein the inner cylinder is rotatably mounted within the outer cylinder, and wherein the target is rotatably mounted directly or indirectly within the inner cylinder.
 2. The deformable image registration phantom of claim 1, wherein the ball and socket mount is rotatably mounted within the housing and has a key extending from the ball and socket mount that fits within a complementary-shaped keyway in the housing, and wherein the key is limited to movement within the keyway in the rotational pitch axis of the phantom.
 3. The deformable image registration phantom of claim 2, wherein the housing has a base with a rotating platform that rotates in the yaw rotational axis of the phantom, and wherein the ball and socket mount is mounted on the rotating platform.
 4. The deformable image registration phantom of claim 3, wherein at least some of the plurality of indexing detents are positioned about the perimeter of the inner cylinder and wherein at least one of the one or more detent pins is positioned to engage with the indexing detents positioned about the perimeter of the inner cylinder.
 5. The deformable image registration phantom of claim 4, wherein the at least one of the one or more detent pins positioned to engage with the indexing detents positioned about the perimeter of the inner cylinder comprises three detent pins, wherein the indexing detents positioned about the perimeter of the inner cylinder are arranged in three rings, each comprising 120 indexing detents arranged at 3° increments, wherein each of the three detent pins are positioned over one of the rings, and wherein the indexing detents in each of the rings are offset from the indexing detents in each of the other rings, such that one indexing detent about the perimeter of the inner cylinder is positioned at every degree increment about the perimeter of the inner cylinder.
 6. The deformable image registration phantom of claim 4, wherein at least some of the plurality of indexing detents are positioned about the perimeter of the outer cylinder and wherein at least one of the one or more detent pins are positioned on the ball and socket mount to engage with the indexing detents positioned about the perimeter of the outer cylinder.
 7. The deformable image registration phantom of claim 6, wherein at least some of the plurality of indexing detents are positioned within the keyway, and wherein at least one of the one or more detent pins are positioned on the key to engage with the indexing detents positioned within the keyway.
 8. The deformable image registration phantom of claim 7, wherein at least some of the plurality of indexing detents are positioned on the rotating platform, and wherein at least one of the one or more detent pins is positioned on the base to engage with the indexing detents positioned on the rotating platform.
 9. A deformable image registration phantom, comprising: a housing; one or more components that move rotationally or translationally relative to the housing or another one of the one or more components; a target mounted to the housing by way of the one or more components; a plurality of indexing detents positioned on at least a first component selected from a group consisting of the housing and the one or more components; and one or more detent pins positioned on at least a second component selected from the group consisting of the housing and the one or more components; wherein the plurality of indexing detents are arranged about at least one rotational or translational axis of the phantom and the one or more detent pins are configured to releasably engage with the plurality of indexing detents as the first and second components rotate or translate relative to one another; and wherein the one or more components are rotatably or translatably mounted within the housing, and wherein the target is rotatably or translatably mounted directly or indirectly within the one or more components. 